1. Field of the Invention
This invention concerns a scanning system for an optical type coordinate input device having a plurality of light emitting elements and photoreceiving elements.
2. Description of the Prior Art
An optical type coordinate input device is disposed at the front of an image display element such as a CRT display or LCD for use in the input of coordinates to a computer. The input device comprises a plurality of light emitting elements and photoreceiving elements disposed opposing to the outer circumference of a CRT display screen or the like and has such a constitution for detecting that light is interrupted by fingers or the likes to thereby obtain coordinate signals during scanning for the light emitting elements.
FIG. 3 is a block diagram showing the structure of the entire circuit of an optical type coordinate input device.
In FIG. 3, when an operation start signal Sc from a not illustrated host computer is inputted, a counter circuit 1 starts the counting operation. Then, the counter circuit 1 issues a pulse signal P to a coordinate detection section 2 and a comparator section 4 on every counting, as well as outputs count data Dc to a memory section 5.
The coordinate detection section 2 comprises, for example, 50 light emitting elements and photoreceiving elements for the detection of X coordinate, as well as 35 light emitting elements and photoreceiving elements for the detection of Y coordinate opposing to each other respectively. Then, on every input of the pulse signal P from the counter circuit 1, since respective light emitting elements are successively scanned by decoders and emit light, it is received by each of the corresponding photoreceiving elements as detection signals X, Y and these detection signals X, Y are outputted.
The detection signals X correspond to pulse signals X.sub.0 -X.sub.49, while the coordinate signal Y corresponds to pulse signals Y.sub.50 -Y.sub.84, for example, as shown in FIG. 4.
These detection signals X, Y are inputted into an amplifier section 3, where they are removed with noise components, waveform-shaped and then amplified to a predetermined voltage level, and then inputted to a comparator section 4.
The comparator section 4 judges whether the detection signals X, Y and the pulse signal P from the counter circuit 1 are synchronously inputted or not. Then, if either of the detection signals X, Y is not inputted at the instance where the pulse signal is inputted from the counter circuit 1, the comparator section 4 outputs a memory signal Sm. That is, if the light from the light emitting element corresponding to the addresses "1" and "51" by a finger or the like and the detection signal X.sub.1 and the detection signal Y.sub.51 (detection signals indicated by the dotted line in FIG. 4) are not obtained, the comparator section 4 outputs the memory signals Sm respectively.
When the memory signals Sm corresponding to the detection signals X.sub.1, Y.sub.51 are inputted, the memory section 5 stores the corresponding count data Dc from the counter circuit 1, that is, the count data Dc indicating "1" and "51". Then, it delivers the count data "1" and "51" to an output control section 6.
When both of these count data "1" and "51" are inputted, the output control section 6 judges that the two count data are suitable as the coordinate signals and then output the two count data as the coordinate signals to the host computer. Accordingly, the host computer judges that the coordinate has been inputted and displays the coordinate on the screen of a CRT display or the like. Then, the host computer continuously outputs the operation start signal Sc to the counter circuit 1 to continuously scan the light emitting elements.
By the way, the counter 1 has been continuously operated so far to thereby continuously scan and emit the light emitting elements in the coordinate detection section 2.
While on the other hand, in the optical type coordinate input device, a plurality of light emitting elements and photoreceiving elements etc are tightly closed within a small casing in order to improve the accuracy for the coordinate detection and the selection operationability, as well as for reducing the size. Accordingly, in the case of continuously scanning and lighting the plurality of light emitting elements as described above, since the temperature within the casing is increased, the operations of the light emitting elements, photoreceiving elements and various kinds of electronic parts become instable due to heating to possibly cause misoperation of the coordinate input device.
Although the temperature rise within the casing is prevented, for example, by perforating vent holes to the casing, dusts or the likes intrude into the casing in this case and they are deposited to light emitting elements, photoreceiving elements or the likes, whereby the coordinate input device also causes misoperation and, depending on the case, coordinate detection itself becomes impossible.